This invention relates to Asynchronous Transfer Mode (ATM) communications, and more particularly to techniques for reducing by many orders of magnitude, ATM cell losses due to burst errors.
The invention is particularly applicable to ATM operation in any telecommunications environment where bit errors on the communications link tend to be bursty, e.g., a coded satellite link whose decoded output is characterized by burst errors.
ATM is a new method for carrying voice, dam, image and video services via telecommunications networks. ATM transports all traffic in fixed-size data units called cells. The ATM cell consists of 5 bytes of overhead (referred to as the cell header) and 48 bytes of data (referred to as the information field). The cell header contains the routing information and is provided with extra protection (so as not to lose 48 bytes of information) consisting of a single bit error correcting ATM Header Error Correction (HEC) code. The ATM cells with two or more bits of error in the header are discarded. This method works very well, resulting in a very low cell loss over communications links with random distribution of single bit errors. Unfortunately, burst errors cannot be corrected by the ATM HEC, resulting in ATM cell losses over bursty error links which can be orders of magnitude higher than over links with random errors. The present invention provides a technique to avoid the loss of cells due to burst error.
In principle, ATM cells may be transported using any digital transmission format. However, the recently approved synchronous digital hierarchy (SDH) and plesiochronous digital hierarchy (PDH) signal formats are of interest from the viewpoint of national and international standardization. An important feature in SDH transmission is the inclusion of pointer bytes, which are used to indicate the location of the first byte in the payload of the SDH frame and also to avoid slips and their associated data loss due to slight phase or frequency variations between the SDH payload and the frame. ATM cell transmission via SDH further incorporates a cell delineation mechanism for the acquisition and synchronization of ATM cells on the receive side of the network.
Transmission considerations and performance criteria for both ATM and its carriage in the SDH and PDH frames are currently based on the assumption that bit errors encountered over the communications path are randomly distributed. The bursty nature of errors results in degradation of performance of both the SDH/PDH transport signals and the ATM cells contained within them through loss of framing. The present invention provides a technique to avoid the loss of framing due to burst errors.
The 48 byte information data field in the ATM cell corresponds to the ATM adaptation layer (AAL). The AAL includes an error detection mechanism which is not robust against burst errors, resulting in undetected errors. Again, the present invention handles burst errors in such a way as not to produce any undetected errors.